First, welcome to the ACA Forums!
Yes, there is an aftermarket for the ACA maps, i.e. eBay (at least the USA version)
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=%22adventure+cycling%22+maps&_sacat=0&LH_TitleDesc=0&_odkw=adventure+cycling+maps&_osacat=0 and here on the Classified section. Just be sure the map is one of the more recent generations. If you go wiht a used (or even new) map, be sure to look for any addendums on the ACA website. Note that you can not share the ACA App like you can the data.
You got the basic idea of the ACA app. Yes, there is an Apple version. You mostly got the idea of the ACA GPX data. You would need a GPS or download it onto your phone. It does come with both the route TRACK and the service data that is shown on the paper maps. A couple of notes: It does not cover all the services out there, i.e. every single convenience store, hotel, etc.; only those on the paper map. Also, i "think" the newer routes do not always include waypoints if you prefer Routes over Tracks. If you were riding with a group of friends, you could buy one set of data and then share the data but please don't sell it or distribute it widely as the sale of the GPX data is copyrighted and is a way to keep ACA funded and it is not ACA's intent for it to be shared.
Hope you have a great ride on the Southern Tier (assuming that one since you are arriving in September).
Tailwinds, John